Unlocking the Interfacial Activity of Insoluble Lithium Salts for Stable Lithium Batteries

Employing functional lithium salts in electrolytes is an effective strategy to suppress anode instabilities and sluggish charge-transfer kinetics in lithium batteries. However, high solubility requirements intrinsically exclude a broad class of promising, high-lattice-energy inorganic salts for carbonate electrolytes. Here, we establish an immobilized functional platform (IFP) by deploying insoluble lithium salts with strong Lewis-acidic Li⁺ sites directly onto an aramid-scaffolded separator. This design bypasses conventional dissolution-diffusion limitations, yielding a thin, ionically conductive, LiF-rich solid electrolyte interphase (SEI) driven by regulated interfacial Li⁺ solvation and enhanced anion-derived decomposition. Systematic screening and ⁶Li/⁷Li isotope tracing identify Li₂SO₄ (LS) as the optimal IFP candidate (IFP-LS) owing to its moderate Lewis acidity, superior solvent affinity, and industrial viability. Remarkably, LS provides a finite sacrificial lithium contribution during continuous SEI evolution, mitigating irreversible lithium consumption. Consequently, IFP-LS separators quadruple the lifespan of Li||LiNi_0.9Co_0.05Mn_0.05O₂ cells at 0.7 C compared to bare separators, and deliver 88.4% capacity retention over 2400 cycles in Li||LiFePO₄ cells at 5 C. Scalability is further validated through 100 m roll-to-roll manufacturing across diverse Ah-level formats, adding only 0.163 USD m^-2. This work overcomes the lithium salt solubility barrier and unlocks a vast library of potent materials for next-generation high-energy-density batteries.